A high voltage flyback transformer is a very economical and reliable means for supplying a high DC voltage for the anode of a cathode ray tube (CRT). Its economy stems from the fact that it derives its high voltage output as a by-product from the operation of a horizontal scan circuit in a raster display such as a television receiver or computer monitor. In such a raster display, the high voltage pulse developed across the magnetic deflection yoke, to return the electron beam from the end of one line to the beginning of the next, is transformed to an even higher voltage and rectified to create a suitable anode voltage for the CRT.
Present-day television receivers and monitors use solid-state circuits with regulated operating voltages. This assures that horizontal and vertical scan currents will remain essentially constant over normal power main voltage variations. Therefore, the flyback voltage generated by the horizontal scan circuit is also essentially constant.
The high-voltage flyback transformer has internal impedances caused by wire resistances and leakage inductance that cause its high-voltage output to drop under load, as is normal with any transformer. In the case of a CRT load, a bright image is a heavy load, while a dark image is virtually no load at all. Therefore, changing image brightness creates a varying load current on a flyback transformer which will cause its voltage to fluctuate. The extent of this voltage fluctuation or "load regulation" is typically 5%
The CRT is an analog device. Every voltage applied to it affects the image in some analog manner. The effect of anode voltage variation is seen primarily as a change in spot deflection distance from center-screen. Electron physics determines that the deflection distance from center is proportional to the inverse square-root of anode voltage. This means that a 5% decrease in anode voltage will produce an increase in deflection of about 2.5%. Or more simply stated, a brighter picture becomes slightly larger as if magnified.
This level of image stability is tolerable in television and character-based displays, so high-voltage flyback transformers have seen wide usage. However, there is a class of displays used for graphics and image processing where image distortion of this type is not permissible. Therefore, means have been sought for voltage regulation of a HV (high voltage) flyback transformer, without reaction on the horizontal scan circuit used to feed it.
U.S. Pat. No. 5,043,598 (the "'598 Patent"), issued Aug. 27, 1991, to Tsutomu Maeda, et al., achieves this goal by means of a series-pass regulator system placed in the high-voltage secondary circuit of a flyback transformer.
It is a general object of this invention to establish certain improvements on the design shown in the '598 Patent.
The design shown in the '598 Patent includes a control circuit having a string of pnp transistors Q.sub.1 -Q.sub.4 (stages) connected together so that the emitters and collectors are connected together in series. The topology shown in the '598 Patent will work up to four or five stages because equal base resistors are provided so as to maintain, at least theoretically, an equal voltage across each transistor stage. Such an arrangement works to a point. However, base currents begin to accumulate in the resistors so that unequal voltages will be created across each stage. Increasing the number of stages therefore accentuates the differences, setting a practical limit on the number of transistor stages that can be used, and therefore a limit on the possible dynamic control of the regulator.
Accordingly, another object of this invention is to improve the topology of the high-voltage transistor string to permit more complete use of each transistor's voltage rating and to make possible strings of up to ten or more stages for greater control voltage range.
The '598 Patent regulator also includes a gap 25 across the string of transistors Q.sub.1 -Q.sub.4 in order to protect the transistors from surges of current. Such gaps, however, exhibit inaccurate breakdown voltages and therefore provide poor protection against surge currents. The gap also is likely to cause severe output voltage transients by misfiring.
Thus, another object of this invention is to provide improved protection of the regulator from damage caused by arcs occurring across the CRT load to ground.
The designs shown in the '598 Patent all require two power supplies for the regulating circuit. It is desirable to reduce the costs associated with the regulating circuit by reducing the parts and designing the circuit so that it operates with a single power supply.
Still another object is to operate such a regulator from a single power supply.
Another desirable feature absent in the '598 Patent is to be able to adapt the regulator circuit so that means are provided for monitoring the anode load current so that the brightness of the CRT image can be automatically controlled or limited.
Accordingly, a further object is to provide a voltage proportional to CRT anode load current for external circuits to sense and use for automatic brightness control or limiting.
It has also been discovered that regulating the output of the flyback transformer can create problems not normally realized with an unregulated flyback transformer. Using the design in the '598 Patent, it was discovered that bright traces appeared occasionally in the CRT display.
After considerable effort, these bright traces were found to be caused by corona discharges from the surface of the plastic case to ambient air, supplied by minute ohmic leakage currents through the encapsulation compound from high-voltage circuits to the case. This effect exists even with excellent insulating materials in the order of 10.sup.14 to 10.sup.16 ohm-cm resistivity. Each corona discharge results in a transfer of charge such that the potential on the plastic case drops sharply by several thousand volts. This potential change is coupled through the capacitance of the insulating material to every part of the regulator housing, including the input to the error amplifier.
Pulses are seen at the error amplifier input with amplitudes often exceeding a volt. Since the error amplifier gain is high, so that errors of not more than a half-millivolt can drive the regulator between zero and full-current output, theses corona-induced pulses are overwhelming to the amplifier and drive it to its saturation limits, until it is able to recover.
Usually, the error amplifier turns off all output current during the recovery interval of several hundred microseconds following a corona pulse. This allows the anode circuit to begin a discharge toward ground along an essentially linear negative ramp of voltage, often able to fall by more than 100 volts before recovering to normal.
In the upper half of the monitor screen, a linear negative ramp on the anode causes scanning lines to bunch together, because of the "inverse square-root of anode voltage" relationship to deflection, causing an apparent "bright trace". The subsequent recovery to normal voltage spreads the lines further apart, creating a "dark trace" which is not as obvious. In the bottom half the reverse is true, with a "dark trace" followed immediately by a "bright trace". At screen center, the effect does not appear.
It is therefore another object of the invention to eliminate output voltage disturbances caused by corona discharges from the regulator's case.
An additional object is to provide automatic programming of the regulated dc output voltage to better accommodate component tolerances in production.
Other objects of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.